The girl who walks out from your shower and sits on your bed in your bathrobe is a stranger. This won't be for long though, as you have other intentions right off the bat. She tells you that she's been all over the place... she hasn't seen ANYONE. Everyone disappeared. It would seem that you two are the only ones left. It's horrifying, but she seems calm about it, having already dealt with this over the course of a week or so being alone.
You insist that the only way forward is to get her pregnant, to repopulate the world in whatever way you can. The whole thing is silly, but you're horny as fuck at this point. She's a virgin, you knew this from the moment you met. What a jackpot - last girl on Earth, meet last man!